1. Field of the Invention
The present invention relates to a method for adaptive Tomlinson-Harashima precoding in a digital data communications link.
The invention also relates to an apparatus for adaptive Tomlinson-Harashima precoding in a digital data communications link and for a receiver.
2. Description of Background Art
FIG. 1 shows one solution according to the state of the art. When transmitting data in digital form, i.e. a bit stream, over a data communications channel 2, the bit stream in question is converted in the transmitter (TX) into an analog signal, which can travel in a transmission channel. The transmission channel can be, for example, a radio path, a copper cable, or an optical-fibre cable. In the receiver (RX), the transmitted bit stream is reconstructed as unchanged as possible, on the basis of the incoming analog signal. The reconstruction of the bit stream in the receiver is hampered by the transmission channel's distorting effect on the signal and by the noise accumulating in the signal. These phenomena lead to errors in some of the reconstructed bits (e.g., on average 1 bit in 107 may be erroneous).
Equalizers, located in either the transmitter, or the receiver, or partly in both, are generally used to compensate the transmission channel's distorting effect on the signal. The equalizers can be either fixed or adaptive. The effect of noise is compensated with various coding techniques, such as Reed-Solomon coding, convolution coding, trellis coding, and turbo coding.
The most generally used method for equalizing channel distortion is an adaptive linear equalizer (FFE). However, on certain channels, using a linear equalizer alone is not enough. This situation arises, if transmission zeroes occur on the signal band, thus preventing certain frequencies from passing over the channel 2. A feedback equalizer is used to compensate the distortion caused by the transmission zeroes. Even in systems, in which the channel 2 does not contain transmission zeroes, the use of a feedback equalizer is often preferable, as it improves the noise tolerance of the system. A feedback equalizer located in the receiver is a decision-feedback equalizer (DFE) while one located in the transmitter is a Tomlinson-Harashima precoder (TML). The system can include both a DFE and a TML. A linear equalizer too can be located in either the transmitter, the receiver, or part in the transmitter and part in the receiver.
In the present publication, the following abbreviations are used in the descriptions of both the prior art and the invention:
CAP Carrierless amplitude and phase modulation
DFE Decision-feedback equalizer
FFE Feed forward equalizer
LMS Least mean square
PAM Pulse amplitude modulation
QAM Quadrature amplitude modulation
RX Receiver
TX Transmitter
TML Tomlinson-Harashima precoder
In the following, a digital data communications link is examined in terms of channel distortion compensation. The line code used is either pulse amplitude modulation (PAM), quadrature amplitude modulation (QAM), or carrierless amplitude and phase modulation (CAP). FIG. 1 shows a model for a system according to the prior art, the receiver 3 of which has an adaptive linear equalizer (FFE) and an adaptive feedback equalizer (DFE) (Lee & Messerschmitt). The channel noise model (CHN) includes fixed filters and possible modulation mechanisms. The bit stream to be transmitted is coded into symbols (S), which are transmitted over the channel 2. In the receiver 3, the output of the channel 2 is processed by equalizers (FFE and DFE), after which symbol decisions (S′) are made from the equalized signal. The symbol decisions (S′) that are made are also referred to as symbols estimated by the receiver. Both adaptive equalizers are adapted to the characteristics of the channel 2 in training performed when a connection is established. In addition, in steady state transmission, the equalizers are also adjusted, to compensate possible variations in the channel 2. The equalizer are adapted and adjusted on the basis of the detection error (e).
FIG. 2 shows a second system according to the prior art (Lee & Messerschmitt). It has an adaptive linear equalizer (FFE) in the receiver 3 and a feedback equalizer (TML) in the transmitter 1. During training, this system too operates like that of FIG. 1, containing a linear equalizer and a decision-feedback equalizer (DFE). Once the training has ended, the values of the coefficient parameters of the decision-feedback equalizer are transmitted, over an auxiliary upstream channel, to the transmitter, where they form a Tomlinson-Harashima precoder (TML). In steady state transmission, the linear equalizer (FFE) is adjusted, but the feedback equalizer of the transmitter (TML) is fixed and is not adjusted.
Tomlinson-Harashima precoding has the advantage over a DFE that, unlike a DFE, precoding does not result in the feedback of the detection error. Particularly if the amplitude response of the channel 2 has a shape that causes large coefficient parameter values to appear in the DFE, feedback of erroneous decisions by the detector is a real problem. In the most serious cases, when using a DFE a single erroneous decision can lead to loss of connection.
Adjustment of the linear equalizer alone is generally sufficient to compensate for the effects of variation in the channel 2. In some cases, the channel 2 includes analog band-stop filters to eliminate, for example, radio interference. The positions of the transmission zeroes of analog band-stop filters may vary when the component values of the filters change with the temperature. Such variation in the characteristics of the channel 2 cannot be compensated by adjusting only the linear equalizer. Another drawback is that the system cannot adapt optimally to changed noise conditions, as the feedback equalizer is not adjusted in steady state transmission.
FIG. 3 shows a method according to the state of the art for solving the problem described above. The system includes a linear equalizer (FFE), a Tomlinson-Harashima precoder (TML), and a decision-feedback equalizer (DFE). Only the FFE and the DFE take part in the training. At the end of the training, the DFE's tap values are transferred to the transmitter, to form the precoder (TML) while the coefficient parameter values of the DFE are reset to zero. In steady state transmission, the FFE and the DFE are adjusted, but the precoder (TML) is not. The advantage of this is that channel and noise-state changes that cannot be handled by adjusting only the linear equalizer are no longer a problem, as the receiver's DFE too is adjusted during steady state transmission. The drawback is the error feedback because of the receiver's DFE. It can be assumed that the coefficient parameter values of the receiver's DFE will remain lower than in the situation according to FIG. 1, because part of the feedback equalization takes place already in the transmitter. Thus the error feedback is also weaker than in FIG. 1. However, the system performance depends essentially on the size of the changes in the characteristics of the channel 2 and in the noise state, compared to the training situation.
The system shown in FIG. 2 or 3 could be directly improved by calculating, in the receiver, the adjustment increments of the coefficient parameters, using the detection error and the symbol decisions, as if adjusting the DFE, but instead to transmit the calculated adjustment increments to the transmitter, over an auxiliary upstream transmission channel. The precoder's coefficient parameter values are updated using the said adjustment increments. The precoder would then also be adjusted in steady state transmission, making the receiver's DFE unnecessary, or allowing the increase in its coefficient parameter values to be limited. However, it can be shown that the adjustment method described above will not work in a general case.
A Tomlinson-Harashima precoder can be updated to correspond to the varying channel characteristics by repeating the following operations: 1) the coefficient parameter values of the feedback equalizer, corresponding to the changed state of the channel, are formed in the receiver, 2) the formed coefficient parameter values, or changes in the coefficient parameter values, are transferred to the Tomlinson-Harashima precoder using, for example, an auxiliary upstream transmission channel.